1. Technical Field of the Invention
The present invention relates to an etching method in which a wafer is directly immersed in an etching solution comprising a mixture of acids for a predetermined time to remove part of the wafer by etching and more particularly, to a wet etching method in order to secure a good flatness of a silicon semiconductor wafer even of a large diameter.
2. Conventional Art
A conventional etching method has been conducted in the following fundamental manner: As shown in FIG. 1A, an etching solution is fed to a vessel 1 from an inlet port 6 formed at a lower portion of the vessel 1 and guided in a space of the vessel through a rectifying plate 2 to form a constant flow velocity (shown by arrows) and plural wafers 3 are held in the etching solution in such a manner that each plural wafer 3 is held with a main surface at a vertical position while being supported by one driving shaft 4 and three support shafts 5 and the plural wafers are etched regularly at a constant pitch along the axial direction, while rotating at a velocity of rotation of tens of rpm. The wafers 3 are immersed into the etching solution in the vessel at one action from over the solution, while rotating in the same condition mentioned above, and thereafter subjected to etching in the solution in the above mentioned manner for a predetermined time in which a target removal by etching is achieved. Thereafter, the wafers are taken out and immediately thereafter, the wafers are again immersed into water to rinse off the etching solution attached to them. Thus the etching treatment process is completed.
The etching solution generally is composed of a mixture of acids (nitric acid, hydrofluoric acid and acetic acid) and water as diluent and it is recovered from a recovery port 8 through an overflow receiver 7 and cooled for recycling.
If an etching solution was an absolutely stationary liquid, a relative velocity of a wafer to the etching solution would be so simple and clear that there are the maximum value at the periphery of the wafer and nothing at the center thereof.
The difference in velocity between the periphery and center automatically results in a difference in cooling effect, and temperature is more rapidly raised as a position on the wafer surface is nearer the center due to reaction heat, so that a removal rate by etching is larger in the central portion. A sectional profile of the wafer after the etching is obtained having a smallest thickness in the central portion.
What's worse, the etching process is hard to be continued due to increase in the temperature of the etching solution, when the process is operated batchwise. Therefore, as is mentioned above in the description of the prior art, in a practical case, the etching solution is fed in the etching process while temperature thereof is controlled at a constant value by some means in order to keep temperature of the etching solution from rising in excess by a reaction heat during etching. However, a feed of the etching solution is generally set at about a fraction of a peripheral velocity of the wafer in terms of a linear velocity in section of the vessel thereof.
FIG. 1B is a diagram showing changes in velocity of three points on a wafer, which are a point at the periphery, a point of a half radius from the center and a point at the center, relative to the etching solution in one rotation of the wafer in a condition that a ratio of a peripheral velocity to a flow velocity of the etching solution is 4 to 1, wherein A, B and C indicate points at the periphery, a half radius from the center and the center respectively, and the ordinate represents the rate of relative velocity and the abscissa represents the angular position of each measuring points.
The velocity at the center is a constant small value, which is indicated by a dotted line. The velocity at each of the periphery and a half radius from the center is both largely changed. The abscissa represents an angular position and at the same time represents an elapsed time. An area contained between a curve and the abscissa shows a distance of travel of each point relative to the etching solution around one rotation of the wafer.
To sum up;
the relative velocity: PA1 the travel distance: PA1 that the vessel comprises: a pair of walls parallel to a plane of rotation of a wafer; and PA1 walls of curved surface, which intersect the pairs of walls at a right angle, and whose centers of curvature are the same, and which are spaced apart from each other along a direction of a radius of curvature with a distance of d therebetween; PA1 that the etching solution is fed from a lower part of the vessel; and PA1 that a flow velocity of the etching solution is adjusted at an arbitrary point between the walls of curved surfaces just before a stream of the solution contacts with a wafer rotating in the etching solution so that the flow velocity is a velocity (r.omega.), which is obtained by multiplying a distance r between the center of curvature and the arbitrary point with an angular velocity .omega. of the wafer or an approximation thereof. PA1 that a space between the walls of curved surfaces in the tank is partitioned by plural curved plates having predetermined curvatures in such a manner that an upper portion of the space of the vessel, which the plural wafers occupy, is kept from the plural partitioning plates so as to allow free rotation of the plural wafers only leaving a narrow margin between the edges of the plural plates and the plural wafers; PA1 that plural partitioned spaces by the plates work as plural solution guides; and PA1 that different flow velocities are respectively adjusted for the solution guides.
periphery&gt;r/2 point&gt;center PA2 periphery&gt;r/2 point&gt;center.
This clearly shows a difference in cooling effects on the wafer between the three points during etching and a cooling effect is decreased at a point closer to the central portion, whereby a thickness is also decreased at a point closer to the central portion since reaction temperature and reaction velocity are increased in combination as cooling effect is decreased. Thus a flatness of the wafer is deteriorated.
However, in order that an influence of rotation of a wafer during etching on flatness thereof is minimized or reduces so as to be neglected, a flow velocity of the etching solution supplied from a lower part of the vessel is required to extremely larger than a relative velocity of the wafer to the etching solution or theoretically large infinitely, which is actually impossible.